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+/* $OpenBSD: drand.c,v 1.1 2011/07/02 18:11:01 martynas Exp $ */
+
+/*
+ * Copyright (c) 2008 Stephen L. Moshier <steve@moshier.net>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/* drand.c
+ *
+ * Pseudorandom number generator
+ *
+ *
+ *
+ * SYNOPSIS:
+ *
+ * double y, drand();
+ *
+ * drand( &y );
+ *
+ *
+ *
+ * DESCRIPTION:
+ *
+ * Yields a random number 1.0 <= y < 2.0.
+ *
+ * The three-generator congruential algorithm by Brian
+ * Wichmann and David Hill (BYTE magazine, March, 1987,
+ * pp 127-8) is used. The period, given by them, is
+ * 6953607871644.
+ *
+ * Versions invoked by the different arithmetic compile
+ * time options DEC, IBMPC, and MIEEE, produce
+ * approximately the same sequences, differing only in the
+ * least significant bits of the numbers. The UNK option
+ * implements the algorithm as recommended in the BYTE
+ * article. It may be used on all computers. However,
+ * the low order bits of a double precision number may
+ * not be adequately random, and may vary due to arithmetic
+ * implementation details on different computers.
+ *
+ * The other compile options generate an additional random
+ * integer that overwrites the low order bits of the double
+ * precision number. This reduces the period by a factor of
+ * two but tends to overcome the problems mentioned.
+ *
+ */
+
+#include "mconf.h"
+
+
+/* Three-generator random number algorithm
+ * of Brian Wichmann and David Hill
+ * BYTE magazine, March, 1987 pp 127-8
+ *
+ * The period, given by them, is (p-1)(q-1)(r-1)/4 = 6.95e12.
+ */
+
+static int sx = 1;
+static int sy = 10000;
+static int sz = 3000;
+
+static union {
+ double d;
+ unsigned short s[4];
+} unkans;
+
+/* This function implements the three
+ * congruential generators.
+ */
+
+static int ranwh()
+{
+int r, s;
+
+/* sx = sx * 171 mod 30269 */
+r = sx/177;
+s = sx - 177 * r;
+sx = 171 * s - 2 * r;
+if( sx < 0 )
+ sx += 30269;
+
+
+/* sy = sy * 172 mod 30307 */
+r = sy/176;
+s = sy - 176 * r;
+sy = 172 * s - 35 * r;
+if( sy < 0 )
+ sy += 30307;
+
+/* sz = 170 * sz mod 30323 */
+r = sz/178;
+s = sz - 178 * r;
+sz = 170 * s - 63 * r;
+if( sz < 0 )
+ sz += 30323;
+/* The results are in static sx, sy, sz. */
+return 0;
+}
+
+/* drand.c
+ *
+ * Random double precision floating point number between 1 and 2.
+ *
+ * C callable:
+ * drand( &x );
+ */
+
+int drand( a )
+double *a;
+{
+unsigned short r;
+#ifdef DEC
+unsigned short s, t;
+#endif
+
+/* This algorithm of Wichmann and Hill computes a floating point
+ * result:
+ */
+ranwh();
+unkans.d = sx/30269.0 + sy/30307.0 + sz/30323.0;
+r = unkans.d;
+unkans.d -= r;
+unkans.d += 1.0;
+
+/* if UNK option, do nothing further.
+ * Otherwise, make a random 16 bit integer
+ * to overwrite the least significant word
+ * of unkans.
+ */
+#ifdef UNK
+/* do nothing */
+#else
+ranwh();
+r = sx * sy + sz;
+#endif
+
+#ifdef DEC
+/* To make the numbers as similar as possible
+ * in all arithmetics, the random integer has
+ * to be inserted 3 bits higher up in a DEC number.
+ * An alternative would be put it 3 bits lower down
+ * in all the other number types.
+ */
+s = unkans.s[2];
+t = s & 07; /* save these bits to put in at the bottom */
+s &= 0177770;
+s |= (r >> 13) & 07;
+unkans.s[2] = s;
+t |= r << 3;
+unkans.s[3] = t;
+#endif
+
+#ifdef IBMPC
+unkans.s[0] = r;
+#endif
+
+#ifdef MIEEE
+unkans.s[3] = r;
+#endif
+
+*a = unkans.d;
+return 0;
+}